Method of preparing niobium base alloy wire



United States Patent I O 3,278,344 METHOD OF PREPARING NIOBIUM BASE ALLOY WIRE Malcolm J. Fraser, Penn Hills, and Raymond E. Wien, Penn Township, Westmoreland County, Pa., assignors to Westinghouse Electric Corporation, Pittsburgh, Ta., a corporation of Pennsylvania No Drawing. Filed July 29, 1963, Ser. No. 298,443 6 Claims. (Cl. 148-11.5)

This invention relates to the preparation of niobium base alloy wire particularly useful for superconducting applications.

Superconducting alloys in substantial lengths of fine wire are desired for many applications. One satisfactory superconducting mateiral is the niobium-zirconium alloy, but it is extremely difficult to produce substantial lengths of wire of that alloy. One of the reasons for this is that contamination adversely affecting formability can occur in many of the steps in preparing the alloy. Secondly, the microstructure of niobium-zirconium alloys makes them extremely susceptible to intergranular cracking. Under these handicaps long lengths of wire cannot readily be obtained.

It is therefore a primary object of the present invention to provide a method whereby niobium base alloy wire can be made in substantial lengths, which method is easily practiced with readily available skills and apparatus.

Another object of the invention is to provide a process whereby niobium-zirconium alloys can be produced having a microstructure that permits the preparation of long lengths of Wire therefrom.

Other objects and advantages of the invention will be apparent from the following detailed description and discussion.

In accordance with the present invention, substantial lengths of niobium base alloy wire can be obtained by applying working and heat treating steps that develop a rnicrostructure in the alloy that can readily be cold worked, as by swaging or wire drawing, to fine wire of long length. In this procedure, the alloy is first hot worked at an elevated temperature where it is substantially ductile. Between working and annealing treatments, in some instances at least, a surface treatment is practiced to insure that the uncontrolled addition of impurities does not occur. In this general manner, we have been able to provide niobium base alloy rod and bar that can be readily shaped to long lengths of wire by cold working without need for intermediate anneals and Without encountering cracking or the like.

A particular utility contemplated for the niobium base alloy wire of this invention is a superconductivity applications. For this utility in particular, it is desirable to control carefully the introduction of impurities into the alloy. Accordingly, the alloy, for example 10 to 65 weight percent, or preferably to 50 percent, of zirconium and the remainder niobium, is made from commerical purity metals. Suitably, bars or rods of these materials are comminuted to a small particle size; then the particles are combined in the desired proportions and pressed to a shape that can be used as a consumable electrode. Pressures on the order of 5 to tons per square inch can be used. If desired, the electrode can be sintered to provide greater strength. Alternatively, a suitable electrode can be provided by forming a sandwich of alternate strips of zirconium and niobium. However formed, the consumable electrode is then are melted to ingot form, suitably 1 to 6 inches in diameter. The are melting is accomplished in a furnace in a vacuum or in an atmosphere of an inert gas, such as argon or the like. Repeated melts can be practiced to improve ingot homogeneity. The ingot of this alloy has a typical columnar 3,278,344 Patented Oct. 11, 1966 microstructure that characterizes arc melted materials, and it is the object of further steps in the present invention to reduce the ingot to a workable size and simultaneously convert its microstructure to an equiaxed structure that can be cold worked readily.

For these purposes, the ingot is first hot reduced, e.g. extruded, to a bar shape of about 0.25 to 1.5 square inches in cross-sectional area. Extrusion also can result in hot tears along the surfaces. Accordingly, it may be useful to protect the surfaces during this cold reduction, as by canning the ingot in a refractory material, e.g. molybdenum, tantalum, tungsten or the like. In canning, the side wall can be slip-fitted over the ingot and end walls can be riveted in place. Extrusion or other hot working is effected above the alloy recrystallization temperature, and suitably at a temperature on the order of 1200 to 1700" C. The bar microstru'cture after extrusion is a bandedsemi-equiaxed structure. It is then cold worked, as by forging or swaging, to a minimum of about 40 percent cold work and preferably 45 to 65 percent. If a canning procedure were used, the can is removed before cold Working. The microstructure still is banded at this point. After the cold working step, a minimum of about 0.01 inch is removed, as by machining, from each surface of the shape.

The banded microstructure is substantially converted to an equiaxed microstructure by placing it in a furnace, in a vacuum or inert atmosphere of argon or the like, at a temperature of about 1000 to 1400 C. for a period of time that may be about 10 minutes to one hour. That is a recrystallization heat treatment. After that heat treatment, the bar can now be cold worked by drawing, rolling or other procedure to any desired final size and shape, e.g. thin wire on the order of 0.001 inch, strip or sheet form.

The invention will be described further in conjunction with the following specific example in which the details are given by way of illustration and not by way of limitation.

Strips of zirconium and niobium, each of a purity of about 99 weight percent, are combined in sandwich fashion in a weight ratio of one part of zirconium to each 3 parts of niobium to form an electrode. The electrode is placed in an arc furnace and, after drawing a vacuum of about 10' mm. Hg, is melted into a two and three quarter inch diameter ingot. The ingot is remelted and recast to aid homogeniety. It is covered with a molybdenum can of inch thickness. Thereupon, the canned ingot is heated to 1600 C. and hot extruded through a die to a wire bar of 1.125 inch diameter. The bar is cooled to room temperature and molybdenum can is ground off. The ingot is cold swaged until its diameter has been reduced to 0.75 inch. The surfaces of the resulting bar are cleaned by machining 0.0001 inch from each surface. The cleaned specimen is then heated in a vacuum furnace at a pressure of 10 mm. Hg at 1300 C. for 30 minutes to cause recrystallization. After cooling to room temperature, the bar is cold swaged to a wire of a diameter of 0.125 inch. This is accomplished without breaking or cracking and without using anneals intermediate passes through the dies. It is then further reduced by drawing to an 0.003 inch diameter.

From the foregoing discussion and description, it will be apparent that this invention comprises a uniquely simple and effective procedure by which niobium base alloy wire can be produced readily in long lengths. The extended lengths result because the alloy shape, by virtue of the working and heating schedules, has a microstructure that permits any degree of cold work without cracking or breaking. The wire obtained can be used in the same manner that similar wire presently is used. It is believed that its best utility is in cryogenic applications,

for example, as a winding for a superconducting solenoid.

While the invention has been disclosed with respect to particular embodiments, it will be appreciated that variations, changes and substitutions may be made without departing from its scope.

We claim:

1. A method of preparing readily cold workable niobium base alloy comprising from 10 to 65 weight percent of zirconium and the remainder niobium, including cold Working a bar of said alloy having an essentially banded microstructure at about room temperature to a minimum of about 40 percent cold Work, then heating it to a temperature above its recrystallization temperature for a period sufiicient to convert it to an equiaxed microstructure.

2. A method in accordance with claim 1 in which the heat treating step in carried out in a vacuum.

3. A method in accordance with claim 1 in which said cold worked bar is heat treated at 1000 to 1400 C. and is conducted in a vacuum.

4. A method comprising melting a consumable electrode of zirconium and niobium containing about 10 to 65 weight percent of zirconium and the remainder niobium to produce a small diameter ingot, hot extruding Wire bar from said ingot to reduce its columnar microstructure to a banded semi-equiaxed microstructure, then cold working the resulting product to a minimum of at least 40 percent cold work, then heat treating the cold worked bar by heating it to an elevated temperature of about 1000 to 1400 C. in a non-oxidizing atmosphere to recrystallize it and result in a readily cold workable equiaxed microstructure.

5. A method comprising melting a consumable electrode of 10 to 65 weight percent zirconium and the remainder niobium to produce an ingot thereof, covering the surfaces of said ingot with a refractory metal, hot working the canned ingot to reduce it to a banded semiequiaxed microstructure, removing the can therefrom and cold working the resulting product to a minimum of 40 percent cold working, cleaning the surfaces of the cold worked bar, then heating the bar to recrystallize it and produce an essentially equiaxed microstructure therein.

6. The method of claim 5 in which the hot working is accomplished at a temperature in the range of l200 to 1700 C. and recrystallization is accomplished at 1000 to 1400 C.

References Cited by the Examiner UNITED STATES PATENTS 5/1962 Redden 14811.5 12/1964 Wong 29-552.2 

4. A METHOD COMPRISING MELTING A CONSUMABLE ELECTRODE OF ZIRCONIUM AND NIOBIUM CONTAINING ABOUT 10 TO 65 WEIGHT PERCENT OF ZIRCONIUM AND THE REMAINDER NIOBIUM TO PRODUCE A SMALL DIAMETER INGOT, HOT EXTRUDDING WIRE BAR FROM SAID INGOT TO REDUCE ITS COLUMNAR MICROSTRUCTURE TO A BANDED SEMI-EQUIAXED MICROSTRUCTURE, THEN COLD WORKING THE RESULTING PRODUCT TO A MINIMUM OF AT LAST 40 PERCENT COLD WORK, THEN HEAT TREATING THE COLD WORK BAR BY HEATNG IT TO AN ELEVATED TEMPERATURE OF ABOUT 1000* TO 14000* C. IN A NON-OXIDIZING ATMOSPHERE TO RECRYSTALLIZE IT AND RESULT IN A READILY COLD WORKABLE EQUIAXED MICROSTRUCTURE. 